Until the recent past, women thought to be pregnant with twins were commonly x-rayed to diagnose the twin pregnancy and to determine the position of the fetuses prior to delivery, generally in the absence of other medical indications for this procedure. This makes twins an especially suitable population for studying the relationship between prenatal x-ray exposure and the subsequent occurrence of childhood cancer. A sample of 97 twin individuals has been identified in the Swedish twin registry, maintained by the National Institute of Environmental Medicine, who have developed cancer before the age of 16 years. This sample is to be supplemented with additional cases through searches of the national death and cancer registration systems. Each case will be matched to two controls selected from the same registry and matched on year of birth, sex of twin, and sex composition of twin pair. Obstetric and pediatric records relating to their birth will be analyzed to meet the objectives of the study.